It is known that continuous application of a forward current to a p-n diode using silicon carbide (SiC) increases a forward bias (for example, see Non-Patent Document 1 below). This occurs because recombined energy obtained when minority carriers injected through the p-n diode are recombined with majority carriers causes the triangular stacking faults (may also be referred to as “Shockley-type stacking faults”), which are plane defects, to extend into crystals from basal plane dislocations, etc. in a silicon carbide substrate as an origin (for example, see Non-Patent Document 2 below). It can be considered that increase in the forward bias of the p-n diode occurs because the triangular stacking faults obstruct the current flow. Such increase in the forward bias may degrade the reliability.
There is a report that such forward bias shift similarly occurs in a metal oxide semiconductor field effect transistor (MOSFET) using silicon carbide (for example, see Non-Patent Document 3 below). In the MOSFET structure, a parasitic p-n diode (body diode) is interposed between a source and a drain, and application of a forward current to this body diode degrades the reliability similarly as the p-n diode. When a Schottky barrier diode chip functioning as a free-wheeling diode and having a low forward bias is connected in parallel with a MOSFET chip, this problem will be mitigated. However, as pointed out by Patent Document 1, the external installation of a diode increases the number of parts included in a device. If the body diode in the MOSFET serves all or a part of the functions of the free-wheeling diode, the reliability of the MOSFET chip may also be degraded as mentioned above.
Examples of a method for addressing this problem include a stress test for applying a forward current to a p-n diode structure for a long time and measuring change in the forward bias before and after the application of the current as mentioned in, for example, Patent Document 2. Rejecting (screening) elements whose degradation is significant in the stress test from products can ensure higher reliability. An amount of variations in the forward bias that is paid attention to in determining the presence or absence of degradation is directly proportional to an area of stacking faults. The expansion rate of this area is almost directly proportional to an integrated quantity of minority carriers injected through a p-n diode. This integrated quantity depends on the magnitude of a current and a time during which the current is flowing. An excess current to finish the test for a short period of time may damage a chip or test equipment through excess generation of heat in a diode element. Conversely, reduction in current requires a longer time to test, which consequently causes practical problems including increase in the chip cost.
On the other hand, semiconductor chips functioning as unipolar transistors such as MOSFETs can include a diode that allows a current to pass only by majority carriers, that is, a unipolar diode as a free-wheeling diode to replace the p-n diode that may degrade the reliability as mentioned above. For example, in Patent Documents 3 and 4, a unit cell of a MOSFET incorporates a Schottky barrier diode (SBD) as a unipolar diode. Inclusion of the unipolar diode having an operating voltage lower than that of a body diode, in a unit cell as an active region of a unipolar transistor can prevent a forward current from flowing through the body diode in the active region in its practical use. Accordingly, degradation in characteristics of the active region can be suppressed.